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Date: Thu, 01 Dec 2016 11:16:00 -0000
From: Jonathan Wakely <jwakely@redhat.com>
To: Tim Shen <timshen@google.com>
Cc: libstdc++ <libstdc++@gcc.gnu.org>, gcc-patches <gcc-patches@gcc.gnu.org>
Subject: Re: [Patches] Add variant constexpr support for visit, comparisons and get
Message-ID: <20161201111615.GC3301@redhat.com>
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On 30/11/16 19:29 -0800, Tim Shen wrote:
>On Wed, Nov 30, 2016 at 8:27 AM, Jonathan Wakely wrote:
>> On 26/11/16 21:38 -0800, Tim Shen wrote:
>>> +  template<typename _Type, bool = std::is_literal_type_v<_Type>>
>>>     struct _Uninitialized;
>>
>>
>> I'm still unsure that is_literal_type is the right trait here. If it's
>> definitely right then we should probably *not* deprecate it in C++17!
>
>No it's not right. We need this only because [basic.types]p10.5.3 (in n4606):
>
>  if it (a literal type) is a union, at least one of its non-static
>data members is of non-volatile literal type, ...
>
>is not implemented. In the current GCC implementation, however, all
>non-static data members need to be literal types, in order to create a
>literal union.
>
>With the current GCC implementation, to keep our goal, which is to
>make _Variadic_union literal type, we need to ensure that
>_Uninitialized<T> is literal type, by specializing on T:
>1) If is_literal_type_v<T>, store a T;
>2) otherwise, store a raw buffer of T.
>
>In the future, when [basic.types]p10.5.3 is implemented, we don't need
>is_literal_type_v.
>
>I'll add a comment here.

Thanks, that will stop me asking again and again in future ;-)

I think we want to get [basic.types] p10 implemented before we declare
C++17 support non-experimental, so we don't have to change
std::variant layout later.

>I didn't check for other compilers.

That's fine, the current approach should work for them too.